Rimfire: Priming charge is located in the rimmed circumference at the base of the cartridge. Small caliber, low-power cartridges. Commonly .22 short, long, long-rifle, .22 magnum. Virtually all other modern ammunition is
Centerfire: Priming charge is in a small cap, inserted into the center of the cartridge base.
Blowback action: a simple mechanical combination in which the breech is kept closed at the moment of firing by the inertia of the slide and breechblock, and by the compression action of the recoil spring (and hammer spring, if present). This pressure counters the reaction impetus of the fired cartridge until the bullet has exited the barrel and the chamber pressure has dropped to a nominal amount, rendering the action safe to open and eject the spent shell. A blowback action is usable for cartridges of moderate power, up through .38, .380, and 9mm Largo. (Since we are talking autoloaders here, standard and exotic revolver cartridges are not part of this description). Cartridges more powerful than these require a
Locked breech: in order to assure that the reaction impetus doesn't cause the breech to open while there is still dangerous pressure in the firing chamber, possibly causing damage to the gun or the shooter. In a locked-breech system typically the barrel, breechblock and slide are solidly, mechanically linked together, and recoil under spring tension for a short distance within the frame. When the unit reaches a predetermined distance, a mechanical contrivance causes the breechblock to "unlock", to be able to travel independently from the barrel and extract, eject, and chamber a new round.
Gun designers have devised many different methods to lock the breech and barrel together and then unlock them, some simple and some excessively complex. It is the breech locking system which makes the Luger unique among almost all other autoloading firearms, gives it part of its distinctive 'look' and its phemonenal accuracy.
The model 1911 .45 Colt is a locked-breech design, but it doesn't operate anything at all like a Luger.
--Dwight
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